Load configuration from file

[aldy505]

To support self hosted users better, and to make sure we don't pollute the environment variables for the Docker Compose file, we really should provide loading configuration from file, while still supporting reading it directly from the environment variables.

One thing why this should be done is to provide better visibility for people who wants to set their blob storage to anything other than Docker volume (ex. gcs, s3, azblob)

A few examples:

• Relay: https://github.com/getsentry/self-hosted/blob/master/relay/config.example.yml
• Sentry: https://github.com/getsentry/self-hosted/blob/master/sentry/config.example.yml / https://github.com/getsentry/self-hosted/blob/master/sentry/sentry.conf.example.py
• Symbolicator: https://github.com/getsentry/self-hosted/blob/master/symbolicator/config.example.yml